At present, wireless home audio/video applications are increasingly numerous. They require payload data rates of the order of 1 Gbps, a continuously rising quality of service and up to ten meters range.
A wireless personal network of mmWave WPAN (“Millimeter Wave Wireless Personal Area Network”) type is particularly well-adapted to this type of application. More particularly, it uses the 60 GHz radio band, in which the wavelengths are of the order of the millimeter, which gives high bandwidth and so enables the transport of a high quantity of data.
The physical properties of the 60 GHz radio band and the legal restriction on power of the transmitters limits communications to a maximum of about ten meters. Indeed, at these frequencies, the degree of reflection on obstacles (walls, furniture, living entities, etc.) is very slight and the degree of attenuation in the air is great. In practice, this makes it necessary for the nodes of a mmWave WPAN to use directional antennae having a high gain.
Advantageously, the various nodes of a mmWave WPAN use so-called “smart antennae” to attain the distances required by the home audio/video applications.
A smart antenna is constituted by a network of radiating elements in an array distribution on a support. This type of antenna enables the implementation of the technique referred to as “beam forming”. According to this technique, the phase and power of each radiating element of the antenna is electronically controlled to obtain a directable transmission and/or reception beam that is narrow to a greater or lesser extent.
Advantageously, when nodes are in transmission mode (denoted Tx), their smart antenna is adjusted to provide a wide transmission beam, in order to be able to reach a maximum number of receiver nodes. When the nodes are in reception mode (denoted Rx), their smart antenna is set to provide a narrow and directable reception beam, in order to increase the gain of the antenna and direct it towards the sending node.
In Rx mode, each node directs its antenna at an angle adapted to the reception of the data coming from a sending node Tx. For each new transmitter, each node in Rx mode must therefore direct its antenna at a new angle adapted to the position of the new Tx node.
The search for the best receiving antenna angle is a major problem in networks using directional antennae. In the case of a network in which the exact positions of the devices are not reliably known, it is often necessary to exhaustively sweep through the all the possible angles in order to choose the best one.
This is all the more the case in systems of mmWave WPAN type, since, due to the wavelengths used and the spatial context, combinations of direct and reflected signals may appear, so creating local energy maxima and minima. It may be that the search for a local maximum will not lead to the optimum antenna angle and it could be useful to sweep through all the possible angles to search for a possible better angle.
The main drawback of this method is that it cannot be applied when a communication is in course, since it leads to checking beam orientations which do not allow reception of the data transmitted by the transmitter considered; it thus leads to a loss of data during that time.
A difficulty to resolve when it is desired to form a communication network of mmWave WPAN type based on a TDMA protocol is to be able to rapidly and reliably indicate the angle that the receiver antenna must adopt for each of the time slots during which the different nodes of the network send.
Precise pointing of the antenna is difficult to maintain since the wavelengths used in these systems are very short and thus the slightest movement of the nodes or a variation in the precision of pointing (for example temperature drift, ageing of the electronics) will affect the reception quality of the radio signal.
It is thus appropriate to produce a means for dynamic tracking or pointing of the antenna beam, such that it enables the best possible reception of the data sent by each transmitter.
The problem of the acquisition and the tracking of the antenna angle for a directional beam is a known problem and several proposals have been made to solve it.
In particular, the document IEEE 802.15-07/942r2 “Beam forming and tracking” issued by the workgroup IEEE P802.15 Wireless Personal Area Network in May 2008 proposes an algorithm for updating the antenna beam. In this proposal, the acquisition of the beam is only made on starting up or when the reception of the data fails. There is no anticipation of the need to re-adjust the angle of the antenna before loss of the data.